/*
 * Get the number of subglyphs of a FT2::GlyphSlot object.
 *
 * Note:
 *   Only valid if the format is FT2::GlyphFormat::COMPOSITE.
 *
 * Examples:
 *   outline = slot.outline
 *
 */
static VALUE ft_glyphslot_num_subglyphs(VALUE self) {